1. Field of the Invention
This invention relates to a vehicle travel control device, and more particularly to a vehicle travel control device which enables a driver to accelerate or decelerate a vehicle as intended when the driver carries out an acceleration operation or a deceleration operation during automatic travel control.
2. Description of the Related Art
For the purpose of making driver's vehicle driving operations easy, a vehicle is installed with a vehicle travel control device for performing automatic travel control, such as constant speed travel control for bringing a vehicle speed to a target vehicle speed and follow-up travel control for causing the vehicle to travel while following a preceding vehicle, namely, adoptive cruise control (ACC). In this vehicle travel control device, an electric control unit (ECU) for automatic travel control calculates a target drive force as a target controlled variable so as to bring the vehicle speed to a target vehicle speed. Moreover, in this vehicle travel control device, the calculate target drive force is output to an engine ECU, and then the engine ECU controls an engine, which serves as a vehicle speed regulating device for regulating the vehicle speed, on the basis of this output target drive force. In such a vehicle travel control device, the automatic travel control is stopped when the driver performs a braking operation.
In recent years, there has been demanded for automatic travel control to be performed at low vehicle speed, e.g., at 10 km/h. In such a vehicle travel control device, when a driver performs a braking operation while the driver's vehicle automatically travels on a slope at low vehicle speed under automatic travel control, the automatic travel control stops. In this case, if the driver cannot stop the vehicle on the slope by means of braking force generated from the driver's braking operation, the position of the vehicle cannot be maintained and thus the vehicle might slide down especially on an ascending slope and behave differently.
Therefore, a technology for preventing the automatic travel control from stopping during the braking operation performed by the drive is proposed in the vehicle travel control device described above. Japanese Patent Application Publication No. 2004-90679 (JP-A-2004-90679), for example, proposes a technology in which automatic travel control is not stopped during braking control performed by a driver, an automatic travel control ECU reduces a target vehicle speed and calculates a target drive force to realize the reduced target vehicle speed, and an engine ECU controls an engine based on the target drive force calculated by the automatic travel control ECU to thereby reduce vehicle speed. Also, in JP-A-2004-90679, the greater the accelerator opening is, the higher the target vehicle speed is corrected, and the greater the brake operation amount is, the lower the target vehicle speed is corrected.
Incidentally, the acceleration/deceleration vehicle speed that is generated by the target vehicle speed corrected as described above is different from the acceleration/deceleration vehicle speed that is generated when an acceleration operation or a brake operation is performed while the automatic travel control is stopped, which brings discomfort to the driver. In other words, inappropriately corrected target vehicle speed cannot provide the driver with acceleration feeling or deceleration feeling that the driver desires. For example, when the target vehicle speed is corrected to a high-speed side more than necessary, the driver feels an acceleration level that is higher than what the driver desires. Also, when the target vehicle speed is corrected to a low-speed side more than necessary, the driver feels a deceleration level that is lower than what the driver desires. Furthermore, when the driver performs the acceleration operation or brake operation roughly, the target vehicle speed is corrected instantly, providing more discomfort to the driver. For example, sudden release of the brake pedal instantly corrects the target vehicle speed to the high-speed side, which accelerates the vehicle suddenly. On the other hand, sudden release of the accelerator pedal instantly corrects the target vehicle speed to the low-speed side, which might decelerate the vehicle suddenly.
An object of the invention, therefore, is to provide a vehicle travel control device which enables a driver to accelerate or decelerate a vehicle as intended without bringing discomfort to the driver, when the driver carries out an acceleration operation or a deceleration operation during automatic travel control.